Senator Robert C. Byrd, a Democrat whose political career has spanned seven decades, announced on Tuesday that he would seek a record ninth term.

"I have the best job in America because I represent you, the people of West Virginia. And I want to keep this job," Mr. Byrd, 87, told several hundred supporters who packed the Capitol Rotunda.

Mr. Byrd served three terms in the House before winning his Senate seat in 1958. He will turn 88 in November and will become the longest-serving senator in American history in June.